Since you experienced a power outage prior to losing internet connection with the router, the router's firmware may have been corrupted. Just look at the LED's at the front of the router and you'll know your router's condition. A blinking power LED or a red DIAG LED means the firmware has been corrupted. Just press the reset button at the back for 10-12 seconds while the router is on as a first step.
***Resetting the router sets it back to factory default settings though.
If the power LED is solid and you don't see any DIAG LED, the router just needs to be reconfigured.
Now to revive internet connection. There are 2 ways to set up the WRT54GS with your Westell modem.
The best way to know which setup will work for you, remove first the router from your network. Connect your main pc directly to the modem and if you need not log in to Bellsouth before you can get online thru the modem, that means the router must be set up to obtain it's IP automatically DHCP. And all you have to do here is to clone the MAC address of your main pc to the router and change the LAN IP address of your router. Here are the steps on how to do this:
a.] Connect your main pc to port #4 at the back of the Linksys router. You just want the pc and the router connected to each other at this point.
b.] Check if #4 is lit at the front of the router. If it is, you may proceed. If it isn't lit, verify your physical connections. Make sure your ethernet cable is connected properly on both ends.
c.]Open your command promt by click Start > Run > type cmd then hit the enter key. On the black window, type ipconfig then hit enter. Look for the default gateway[usually 192.168.1.1].
c.] Access the setup page of the router by typing the value for your default gateway[usually 192.168.1.1] on your browser's address bar. A username and password window should appear.
Leave the username blank then type admin for the password. Click OK and you will then arrive at the setup page.
*If you cannot get to the username and password window, If you are using Internet Explorer, try disabling any firewall program that you have installed on your pc and check on your browser settings. Click "tools" > "internet options" > on the connections tab, "never dial a connnection" should be selected. Click on "LAN settings" and no option should be selected there. Apply those settings then go to the privacy tab and have the pop-up blocker disabled if you have it installed.
Apply the settings then try accessing again the default gateway[usually 192.168.1.1] ***I sound like a broken record. =)
d.]When you're inside the setup page already, click the sub tab for MAC address clone. Enable it then click the clone MAC button. Click save settings at the bottom of the page afterwards. Click continue.
e.]Click on the basic setup sub tab then change the LAN IP address of the router from 192.168.1.1 to 192.168.1.253 then click save settings at the bottom of the page. You may get a message after a few seconds. Just allow the page to close if you see any message.
f.]Unplug the power from the router. Unplug the power from the Westell modem. Shut down your pc. Wait for 1 minute.
g.]Connect the Westell modem to the internet port of the router then power up your modem. Wait for 2 minutes then power up the router. Wait for the power LED to be solid on the router. Boot up your pc then try going online.
If you still can't get online, access the setup page fo the router again. You need to use the new IP address of the router though. Type 192.168.1.253 on your browser's address bar. Again, leave the username blank and type admin for the password. Set up the router for PPPOE by following this link from the Linksys Knowledge Base:
http://linksys.custhelp.com/cgi-bin/linksys.cfg/php/enduser/std_adp.php?p_faqid=2210
I really hope this helps you. Cheers! =)